Lithium secondary batteries and other nonaqueous secondary batteries are in increasing demand as power sources in vehicles using electricity as the drive source, and as power sources in personal computers, handheld devices and other electrical products and the like. Lithium-ion batteries hold particular promise as high-output vehicle-mounted power sources because they provide high energy densities with low weight. Patent Documents 1 through 6 are examples of documents of prior art in the field of nonaqueous secondary batteries.    Patent Document 1: Japanese Patent Application Laid-open No. 7-220759    Patent Document 2: Japanese Patent Application Laid-open No. 2004-134244    Patent Document 3: Japanese Patent Application Laid-open No. 2003-132889    Patent Document 4: Japanese Patent Application Laid-open No. 119-320590    Patent Document 5: Japanese Patent Application Laid-open No. 2005-310764    Patent Document 6: Japanese Patent Application Laid-open No. 2007-103066
Typical electrodes (positive and negative electrodes) in lithium-ion batteries are composed of a layer (active material layer) that consists primarily of a material (active material) capable of reversibly storing and releasing lithium ions, and that is supported on a collector. A preferred method of forming this active material layer is to disperse or dissolve the active material in powder form (active material powder) in a suitable solvent to prepare a paste or slurry composition (active material layer-forming composition) that is then applied to a collector, dried, and then pressed as necessary.
Providing a porous layer consisting primarily of inorganic particles on the surface of the active material layer is an effective means of increasing the reliability of lithium-ion batteries and other batteries (by preventing internal short-circuits and the like). From the standpoint of productivity and the like, this porous inorganic layer can be preferably be formed by dispersing the inorganic particles in a liquid medium comprising a binder dissolved in a suitable solvent to form a paste or slurry composition (coating agent), which is then coated on the surface of the active material layer. Patent Document 1 describes a nonaqueous electrolyte secondary battery wherein a porous protective film consisting of a resin binder and solid particles is formed on the surface of the active material layer. The porous protective film can be formed by coating a fine-particle slurry consisting of fine particles dispersed in a solvent together with a binder.
When such a coating agent is coated on the surface of the active material layer, at least some of the liquid medium seeps into the active material layer. The binder contained in the liquid medium covers the surface of the active material particles, which can detract from the input-output performance of the battery. Forming the inorganic layer in such a way as to control this loss of input-output performance is particularly significant in the case of batteries for vehicular power sources, which require high input-output performance.